Many planetary transmissions have upshift and/or downshift conditions before and after which the same planetary gear set provides the drive ratio, i.e., underdrive/direct drive or direct drive/overdrive. During the interchange or shift sequence, it is desirable to avoid both friction device overlap and engine flare. The overlap condition occurs when two friction devices are engaged with sufficient capacity to simultaneously transmit a significant amount of torque. This results in a torque sag which is noticeable by the operator. Engine flare occurs when neither friction device has sufficient capacity to transmit the engine torque. This results in an engine revving condition and slipping of at least one of the friction devices.
In most planetary type transmissions, the ratio interchange is controlled by at least one one-way torque transmitting device, such as a clutch or brake. The one-way torque transmitter will release only when the oncoming device has sufficient capacity to transmit the required torque or will engage when the off-going device can no longer support the torque level. When one of the torque transmitting devices has a band brake which is engaged by a fluid operated servo, a "wash out" shift can be employed. With this arrangement, the pressure at the on-coming device is also directed to a release chamber in the servo. Thus, as the on-coming device gains torque capacity, the off-going (band brake) device loses capacity. This permits a smooth ratio interchange under most conditions.
With the advent of electronically controlled transmissions, the management of friction device overlap is given to a computer or central processor (CPU), which can control the pressure at each individual device. This will permit, with the engineers term, a "clutch-to-clutch" shift. That is, both the on-coming and off-going devices are of the disc type whether they are employed as either a clutch or brake.
However, the electronic controls do add to the cost and complexity of the transmission, such that many transmission manufacturers continue to use one-way devices and "wash out" sequencing to control the ratio interchange while using a CPU and electronic controls to establish control pressures and shift point regulation.
The use of electronic controls for pressure control is, of course, advantageous in that adaptive type controls can be utilized. That is, the control pressures can be modified depending upon the outcome of the previous events. For example, if a shift is too harsh, the electronic control will be informed of that fact and will, under the same shift conditions, reduce the rate of pressure increase to the in-coming device until the desired shift smoothness is achieved.